Ventricular fibrillation, an often fatal heart arrhythmia, can be terminated by the application of one or more electrical current pulses delivered to the heart through electrodes applied to the chest or implanted within the body. Since the first use on humans of a completely implantable cardiac defibrillator in 1980, research has focussed on making continually smaller and more efficient defibrillation devices. In addition, reducing the defibrillation threshold (DFT) energy level applied to the heart by the defibrillation pulses reduces the likelihood of damaging tissue adjacent the electrodes.
A conventional implantable defibrillator includes an electrical pulse generator and an arrhythmia detection circuit coupled to the heart by a series of two or more electrodes implanted in the body. A battery power supply, and one or more charge storage capacitors are used for delivering defibrillation shocks in the form of electrical current pulses to the heart.
Currently, the primary constraint in reducing the size of an implantable defibrillator is reducing the battery size and the size of the storage capacitor(s). Accordingly, improvements in the area of implantable defibrillators have focussed in two areas: (1) more efficient defibrillation waveforms, and (2) more efficient electrode configurations and placements. Stated in other words, the primary variables that can be adjusted in the design to lower the shock strength required for defibrillation include those variables relating to the defibrillation waveform, such as duration, polarity, and waveshape, and those variables relating to the electrodes, such as materials, size, shape, and location.
An example of a development in the area of electrodes is U.S. Pat. No. 4,827,932 to Ideker et al. which relates to a pair of spaced apart epicardial implantable defibrillation patch electrodes. A respective patch electrode is attached over each of the right and left ventricles in an attempt to achieve a uniform voltage gradient throughout the entire ventricular mass.
In the area of defibrillation waveforms, U.S. Pat. No. 4,641,656 to Smits discloses a method of applying a sequence of defibrillating pulses to the heart from a series of four electrodes. Two adjacent electrodes have positive polarity and the other two electrodes have negative polarity in an attempt to concentrate defibrillation energy in the heart wall rather than through the center of the heart. Two or more such pulses are applied, with a reverse in polarity of one pair of opposing electrodes between each pulse. Another pulsing scheme is disclosed wherein the polarity of the four electrodes alternates with each adjacent electrode, and with all four electrodes used simultaneously to defibrillate the heart.
Other examples of defibrillating waveforms are disclosed in U.S. Pat. No. 4,637,397 to Jones et al., No. 4,800,883 to Winstrom, and No. 4,821,723 to Baker, Jr. et al. These patents disclose multiphasic defibrillation waveforms wherein the polarity of pulses is reversed. U.S. Pat. No. 4,768,512 to Imran relates to a high frequency truncated exponential waveform. U.S. Pat. No. 4,727,877 to Kallok discloses a transvenous lead configuration wherein a first electrical pulse is delivered to a first pair of electrodes between the right ventricular apex and the superior vena cava, and after a predetermined delay, a second pulse is delivered to a second pair of electrodes between the right ventricular apex and the coronary sinus.
None of these efforts, however, sufficiently control the waveform to maximize the efficiency of the defibrillation pulses and thereby reduce the risk of damage to adjacent tissue and minimize the size of batteries, capacitors and other defibrillator hardware.